ANDERSEN AIR FORCE BASE, Guam: To take part in annual exercises with the U.S. Air Force last month, Japan practiced dropping 500-pound live bombs on Farallon de Medinilla, a tiny island in the western Pacific about 240 kilometers north of here.

The pilots described dropping a live bomb for the first time, shouting "shack!" to signal a direct hit and seeing the fireball from aloft. "The level of tension was just different," Captain Tetsuya Nagata said as he stepped down from his cockpit onto the tarmac.

The exercise would have been unremarkable for almost any other powerful military, but it was highly significant for Japan, a country still restrained by a Constitution that renounces war and allows forces only for its defense. Dropping live bombs on land had long been considered too offensive, so much so that Japan does not have a single live-bombing range.

Flying directly from Japan and practicing live-bombing runs on distant foreign targets would have been regarded as unacceptably provocative because the implicit message was clear: These fighter jets could perhaps fly to North Korea and take out some targets before returning home safely.

But from here in Micronesia to Iraq, Japan's military has been rapidly crossing out items from its list of can't do's.

The incremental changes - especially since the Sept. 11, 2001, attacks on the United States - amount to the most significant transformation in the Japanese military since World War II, one that has brought it ever closer operationally to America's military while rattling nerves throughout northeast Asia.

In a little over half a decade, the Japanese military has carried out changes considered unthinkable a few years back. In the Indian Ocean, Japanese destroyers and refueling ships are helping the U.S. military fight in Afghanistan. In Iraq, Japanese planes are transporting cargo and U.S. soldiers to Baghdad from Kuwait.

Japan is acquiring weapons that blur the line between defensive and offensive. For the Guam bombing run, Tokyo deployed its newest fighter jets, the F-2, the first developed jointly by Japan and the United States.

Unlike its older jets, the F-2s were able to fly the 2,700 kilometers, or 1,700 miles, from northern Japan to Guam without refueling - a "straight shot," as the Japanese military said with pride.

Now Japan is indicating that it is intensely interested in buying the F-22 Raptor, a U.S. stealth fighter known mainly for its offensive capabilities, like penetrating contested airspace and destroying enemy targets, and whose export is prohibited by U.S. law.

In Tokyo, the Defense Agency, whose profile had been kept low, became a full ministry this year. Prime Minister Shinzo Abe used the huge parliamentary majority he inherited from his popular predecessor, Junichiro Koizumi, to ram through a law that could lead to a revision of the pacifist Constitution.

Japan's 241,000-strong military, although smaller than those of its neighbors, is considered Asia's most sophisticated. Its $40 billion military budget has ranked among the world's top five in recent years. Tokyo also has tapped nonmilitary budgets to launch spy satellites and strengthen its coast guard.

Critics say Abe and other Japanese politicians have sought to justify the military's transformation by seizing on the threat from North Korea; the rise of China, whose annual military budget has been growing by double digits; and the Sept. 11 terrorist attacks. At the same time, he has tried to rehabilitate the reputation of Japan's imperial forces by whitewashing their crimes before and during World War II, including massacres and sexual slavery.

Critics at home say the military changes under way, details of which the government has tried to hide from public view, especially for missions in Iraq, have already violated the Constitution and other restrictions.

"The reality has already moved ahead, so they will now talk about the need to catch up and revise the Constitution," said Yukio Hatoyama, secretary general of the main opposition Democratic Party.

Richard Samuels, a Japan expert at the Massachusetts Institute of Technology, said that revisionist politicians like Abe and Koizumi, once on the fringes of Japan's political world, have succeeded in grabbing the mainstream in a time of uncertainty. They shared the view, Samuels said, "that the statute of limitations on Japan's misbehavior during the Pacific War had expired" and that Japan should have a military like any normal country.

Their predecessors feared getting entangled in a U.S.-led war. But the new leaders feared that Japan would be abandoned by the United States unless it contributed to its wars, Samuels said.

"So what do you do?" he asked. "You step up. And that is consistent with what they've long wanted to do anyway. So there was a convergence of preferences."

Japan today is America's biggest partner in developing and financing a missile defense shield in Asia. Some Japanese ground and air force commands are also moving inside U.S. bases in Japan so that the two forces will become, in military jargon, "interoperable."

"I think the Japan-U.S. security relationship should be as unified as possible and our different roles need to be made clear," said Shigeru Ishiba, a defense chief under Koizumi and now a leader in a Liberal Democratic Party committee looking at loosening defense restrictions.

In Iraq, in accordance with a special law to aid in reconstruction, a symbolic ground force was first deployed to a relatively peaceful, noncombat area in the south to engage in humanitarian activities. After the troops left last year, though, three Japanese planes began regularly transporting U.S. troops and cargo from Kuwait to Baghdad.

The Japanese authorities decline to say whether the planes have carried weapons besides those carried by soldiers. Concerned about public opposition, defense officers have spied on anti-war advocates and journalists perceived as critical, the Defense Ministry acknowledged after incriminating documents were recently obtained by the Communist Party.

Hatoyama of the Democratic Party said that transporting armed U.S. troops contravened Japan's Constitution. "Instead of engaging in humanitarian assistance, they are basically assisting American troops," he said. "American troops and the Air Self-Defense Forces are working as one, just as they are training as one in Guam."

In Parliament, Abe said the activities did not violate the Constitution, saying Japanese troops were restricted to noncombat zones and did not operate under a joint command with any other force.

In Guam, U.S. and Japanese pilots simulated intercepts and air-to-air combat over two weeks. In the final days, each side took turns pummeling the tiny island with bombs weighing the equivalent of 225 kilograms.

Colonel Tatsuya Arima, commander of the Japanese squadron, said that bombing could protect Japanese ground troops or vessels. "Bombing does not always mean offensive weapons," Arima said. "They can also be used for defense, which, put another way, is what we mostly train for."

Lieutenant Colonel Tod Fingal, commander of the U.S. squadron, said the exercise helped to build up confidence among pilots by exposing them to a new environment. "I would equate it to an away game in sports," Fingal said.

Japan's military has become less shy in projecting its power away from home. Japan lacks the nuclear submarines, long-range missiles or large aircraft carriers that amount to real power projection.

But it is acquiring four Boeing 767 air tankers that will allow its planes to refuel in midair, as well as two aircraft carriers that will transport helicopters and, with some adjustments, planes capable of taking off vertically.

The United States has welcomed the changes while pressing for more.

"The restrictions that Japan has lived under, which I would say Japan has maintained on its own or imposed on itself, are quite unique," said a Pentagon official who requested anonymity so that he could speak candidly. "The changes that you're seeing in Japan are very unique changes in the context of those restrictions. In the context of everything else that is going on around the world, or in the context of Japan's potential to contribute to the region and the world in security areas, the changes are fairly small."

Small or not, they are causing anxieties in a region where distrust of Japan has deepened in direct proportion to Japanese tendencies to revise the past.

South Korea, for one, reacted sharply to Japan's desire to buy the F-22 Raptor.

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